Indirect/direct evaporative coolers (IDEC) are two stage evaporative coolers that are systems deploying “sensible cooling” (described below) without moisture addition in the first stage and evaporative cooling in the second stage.
Sensible cooling is a process by which air is cooled without any change in the absolute humidity or level of water vapour. There is neither any gain nor loss of absolute humidity. As shown in the conventional psychometric chart, FIG. 1, the process of cooling of primary air is shown from point A to Point B. Such sensible cooling is achieved by having another stream of secondary air taking the path from C to D. As the secondary air stream is made to flow in alternate wet channels having thin ‘water film’ formed over treated surface, it collects moisture due to vaporisation of water film and a rise in its temperature. This process is depicted from point C to D in FIG. 1.
IDEC can substitute conventional vapor compression indoor air conditioning systems in dry to moderately dry climates because of three primary advantageous features:    1. Energy Efficient            IDEC units do not use a refrigerant compressor and are thus significantly more energy efficient than conventional air conditioning systems.            2. Higher Cooling Capacity            Further, IDEC units being a two-stage evaporative system in comparison to conventional single stage (i.e. direct) evaporative coolers can have more cooling capacity.            3. Reduced Moisture            IDEC units add less moisture to the conditioned space thereby providing better indoor environment and comfort of occupants.        
All the above three assist in mitigating the environmental damage associated with conventional air conditioning systems.
Indirect evaporative cooling is typically accomplished by passing two air streams through a system comprising thin parallel heat exchange plates with alternating dry and wet passages. Primary airstream, to be cooled, is passed through the dry passages; simultaneously, a secondary airstream is passed through the parallel wet passages. The available temperature gradient between the primary and secondary air streams on either sides of the thin parallel plate drives the heat flow from primary air side to secondary air side. The secondary air stream comes in direct contact with the water film formed due to fine spray of water on the surfaces of the heat exchange plates on wet side. Water film vaporises by extracting the required latent heat there by cooling the plates. Thus the primary air stream is cooled by contact with the dry surfaces of the cooled plates, as heat is transferred from the warmer dry side to the evaporatively-cooled wet side. In a typical IDEC design, the primary airstream is further cooled, downstream of the indirect evaporative stage, in a direct evaporative cooling stage before entering a building as cool supply air.